The metering sensor is under the mirror and is used to set aperture and shutter speed. This means light is getting to the metering sensor. Obviously, if it gets to the metering sensor, then it probably gets to the main sensor as well, its just not very much light.

The metering sensor was under the mirror in old designs, but these days it's actually up in the pentaprism housing (labeled in the upper right of this diagram taken from a Canon tech report):

So, light that reaches the metering sensor should be blocked from hitting the image sensor by the mirror when it's flipped up during the exposure.

+1

OOPS, its the AF sensor down there. I knew that, just getting too old.

The metering sensor is under the mirror and is used to set aperture and shutter speed. This means light is getting to the metering sensor. Obviously, if it gets to the metering sensor, then it probably gets to the main sensor as well, its just not very much light.

The metering sensor was under the mirror in old designs, but these days it's actually up in the pentaprism housing (labeled in the upper right of this diagram taken from a Canon tech report):

So, light that reaches the metering sensor should be blocked from hitting the image sensor by the mirror when it's flipped up during the exposure.

+1

OOPS, its the AF sensor down there. I knew that, just getting too old.

Had a couple of noggins tonight guys so maybe not thinking to straight but what then does that piece of rubber crap that goes over the eye piece do for things thats attached to the camera strap?

The metering sensor is under the mirror and is used to set aperture and shutter speed. This means light is getting to the metering sensor. Obviously, if it gets to the metering sensor, then it probably gets to the main sensor as well, its just not very much light.

The metering sensor was under the mirror in old designs, but these days it's actually up in the pentaprism housing (labeled in the upper right of this diagram taken from a Canon tech report):

So, light that reaches the metering sensor should be blocked from hitting the image sensor by the mirror when it's flipped up during the exposure.

100% confirmed - for my 5D3. I have done these crazy tests and most importantly - every frame I exposed (using manual focus) came out completely black. Therefore, no stray light from the “metering leakage” is getting to the sensor. I even did a 120 second bulb exposure with direct sunlight onto the LCD panel at ISO800 and only the slightest stray light was witnessed and that was found to come through the VF cover – so it isn’t perfect either. When I removed this tiny VF leak, a 256 second bulb exposure got absolutely nothing. To put in context, a proper exposure was 1/1000 second at f/8.

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canon rumors FORUM

Oh no..i think we have a bigger design flaw here than just the 5D3 light leak..After all the tests done, i think the biggest flaw is….OUR thumbs leak light!!!So all those years of covering someone's eyes with our hands proved to be useless!! They can still see with light leaking through!!! Oh no…wonder if we will be recalled.

nickzh

I just did a similar test, taking two outdoor pictures with my new 5D MK iii. The one without the lcd light on is metered for 10" exposure duration and turns out fine. When I use the lcd backlight the meter shows 6" exposure duration and the shot gets underexposed by almost one 1 stop.I acknowledge these tripod mounted night shots with long exposure duration are not the ones I do regularly, but to me the light leak problem seems very real. So, should I send the camera back?

I just did a similar test, taking two outdoor pictures with my new 5D MK iii. The one without the lcd light on is metered for 10" exposure duration and turns out fine. When I use the lcd backlight the meter shows 6" exposure duration and the shot gets underexposed by almost one 1 stop.I acknowledge these tripod mounted night shots with long exposure duration are not the ones I do regularly, but to me the light leak problem seems very real. So, should I send the camera back?

Is it normal practice (for you or anyone else for that matter) to have the LCD backlight on when metering? If your answer is no, then just use it as you normally would and don't waste your time and potentially money returning it. If yes, then may be you need to review your practices and/or make manual adjustments.

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Canon 5D MkIII, 7D, 300mm L IS f/2.8 and a few other L's

canon rumors FORUM

If anything is used outside of its intended function, it will probably result in some unintended or unusual outcome. For example, if the reverse gear is used to try stop a car, this will result in a less than desirable driving experience. In one of the tests done by a user, a long exposure was taken at night on a tripod and when the LCD light was turned on, the exposure time decreased. Perhaps, the LCD display needed to be lighted up to check the settings, but after it turns off automatically in a few seconds, then an exposure can be taken. There is no need for the LCD to be lighted up when an exposure is taken. For long exposures, it is common practise to cover the camera body with a light-proof black cloth to prevent stray light from entering the VF and causing flaring effects in the exposure. I doubt Canon will do anything about this "issue" and at the most Canon will just send out an application note?

Perhaps, the LCD display needed to be lighted up to check the settings, but after it turns off automatically in a few seconds, then an exposure can be taken. There is no need for the LCD to be lighted up when an exposure is taken.

But what if you wanted to repeatedly verify your settings during the exposure by viewing the LCD during the exposure?!? Don't go saying that's not realistic because you can't change the settings at that point or that pressing a button on the camera would result in a blurry image. It's more realisitc than taking pictures with no lens...